1. Field of the Invention
The subject invention relates to a hub seal, and more particularly, to a hub seal having a machinable thrust ring which reacts to axial loads imposed during installation of the hub seal on the wheel end of a vehicle, and which is partially removed through shearing action during the break-in period of the seal. The hub seal includes a lay-down lip seal formed from an elastomeric material.
2. Background of the Related Art
In tractor and trailer wheel end axle assemblies, particularly those employed on large trailers, semi-trailers and tractors, the wheel is mounted on a fixed axle for rotation thereabout. The hub of the wheel defines a lubricant chamber about the end of the axle in association with the wheel bearings. A quantity of lubricant is maintained in the lubricant chamber to continuously bathe the bearings. A hub cap is used to enclose the lubricant chamber and a shaft seal is fit onto the axle to effect a dynamic seal between the axle and the lubricant chamber.
Shaft seals are well known in the art, and generally include a wear member or sleeve and a seal member or casing. The casing is mounted to rotate with the wheel hub relative to the sleeve member. The casing has an outer diameter which is designed to provide an interference fit with the wheel hub and the sleeve is dimensioned to be press fit onto the wheel axle. An early example of a unitized shaft seal in which the wear member and seal member are constructed as a one-piece assembly is disclosed in U.S. Pat. No. 3,685,841 to Keller. Later examples of semi-unitized shaft seals in which the wear member and seal member can be disassembled for repair and inspection are disclosed in U.S. Pat. Nos. 4,208,057 and 4,252,329 to Messenger.
It is known in the prior art, as illustrated in each of the above-identified patents, to provide the seal member with elastomeric bumper elements that serve to maintain the axial spacing of the wear member and seal member, as well as absorb the axial load exerted upon the seal member when the seal assembly is press fit onto the axle shaft. Upon installation, the bumper elements are compressed and during the initial break-in period of the seal assembly, the bumper elements are partially worn away to provide a minimal running clearance between the wear member and the seal member.
During the break-in period, the elastomeric bumper elements generate significant resistance to rotation between the sleeve and casing. This resistance generates heat and must be overcome by increasing the torque applied between the sleeve and casing. The increased heat can have an adverse effect on the integrity of the seal and the increase in torque has an adverse effect on vehicle efficiency. In addition, the particulate material worn away from the bumper elements can contaminate the bearing chamber and cause damage to the seal assembly.
U.S. Pat. No. 5,015,001 to Jay discloses a seal assembly which includes an annular bumper element formed from a fusible material which transitions via frictional heating from a solid to a liquid during the break-in period. During break-in, it can take a considerable amount of time and applied torque to sufficiently elevate the temperature of the bumper element to a induce a phase change. This can have an adverse effect on vehicle efficiency. Clearly, there is a need in the art for a hub seal of the type having a bumper element that can be removed during the break-in period without causing an increase in torque and without generating excessive heat.
It is also known in the art of hydrodynamic shaft seals to provide a radial lip element in sealing contact with the axial sealing surface of a wear sleeve to prevent lubricating fluid from migrating along the sealing surface. In general, there are two types of radial lip elements employed in shaft seals. The first type provides one or more thin bands of contact between the sealing lip and the sealing surface of the wear sleeve. Examples of such seals are disclosed in U.S. Pat. No. 4,695,063 to Schmitt et al., U.S. Pat. No. 4,844,480 to Gralka, and U.S. Pat. No. 4,906,009 to Saitoh and U.S. Pat. No. 5,427,387 to Johnston.
The second type provides a deformable lip element which, upon installation, is deflected from its normal orientation into sealing contact with a circumferential area of the axial sealing surface of the wear sleeve. Deformable radial lip seals of this type are commonly referred to as "lay-down" lip seals and are typically fabricated from a synthetic resin material such as polytetrafluoroethylene (PTFE). Examples of such seals are disclosed in U.S. Pat. No. 4,591,168 to Holzer and U.S. Pat. No. 5,209,499 to Ruff et al.
When a sealing lip fabricated from a material such as PTFE is deformed during assembly and subsequently installed in an operating environment in which it is subjected to significant mechanical stresses, material degradation and a loss of resiliency can occur rapidly. It would be desirable therefore, to increase the service life of a hydrodynamic shaft seal by providing a lay-down lip seal fabricated from an elastomeric material which could be biased into sealing engagement with the sealing surface of the wear sleeve. It would also be desirable to provide such a lip seal with structures to increase the hydrodynamic pumping effect of the sealing element. Examples of lip seals with such structures are disclosed in U.S. Pat. No. 4,441,722 to Pichler and U.S. Pat. No. 4,783,086 to Bras et al.